High intensity fluorescent lamp radiating ionic radiation within the range of 1,600{14 2,300 a.u.

ABSTRACT

Very high intensity fluorescent lamp apparatus includes a lamp envelope having a coating therein of a phosphor which emits visible radiation when excited by far ultraviolet ionic radiation. Interior of the envelope, a far ultraviolet radiation source includes a pair of discharge electrodes and a suitable pressure of a vaporizable, ionizable metal together with a partial pressure of noble gas, the metal and noble gas being sufficient, under excitation of a high current density discharge to produce far ultraviolet ionic radiation at current densities of approximately one ampere per square centimeter or greater.

United States Patent 1191 Johnson HIGH INTENSITY FLUORESCENT LAMP RADIATING IONIC RADIATION WITHIN THE RANGE OF 1,6002,300 A.U.

Peter D. Johnson, Schenectady, N.Y.

General Electric Company, Schenectady, N.Y.

Filed: Oct. 31, 1972 Appl. No.: 302,582

Related US. Application Data Continuation-impart of Ser. Nos. 80,896, Oct. 15, 1970, abandoned, and Ser. No. 80,901, Oct. 15, 1970-, abandoned.

US. Cl 3131109, 313/185, 313/225 1m. (:1. 11013 1/62 Field of Search 313/44, 108, 109,

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 10/1939 Ruttenauer 313/109 1 Dec. 11, 1973 2,362,385 11/1944 Libby 313/109 2,473,642 6/1949 Found et a1 313/225 X 3,085,171 4/1963 Smialek 313/184 X 3,525,864 8/1970 Leach 313/109 X Primary ExaminerPaul L. Gensler Attorney-John F. Ahern et a1.

[57] ABSTRACT 10 Claims, 1 Drawing Figure HIGH INTENSITY FLUORESCENT LAMP RADIATING IONIC RADIATION WITHIN THE RANGE OF 1,600-2,300 A.U.

This application is related to U.S. Pat. Nos. 3,657,590, 3,657,591, and 3,679,928, all filed June 26, 1970, and is a continuation in part of my co-pending applications, Ser. No. 80,896 and Ser. No. 80,901 (now abandoned), both filed Oct. 15, l970, all of which were co-pending applications, are assigned to the assignee of this invention, and incorporated herein by reference thereto.

This invention relates to electric lamps in which luminescent phosphors are excited to the emission of visible and near ultraviolet radiation when excited by far ultraviolet radiation.

Fluorescent lamps of the prior art are generally designed to operate upon the mechanism of excitation of visible light emitting phosphors by a low current mercury discharge. Because of the parameters of the mercury discharge utilized, the radiation therefrom is essentially concentrated in the ultraviolet spectrum and is constituted principally of the characteristic 2,5 37 A. U. mercury atomic emission line. Although the foregoing radiation is sufficient to excite fluorescent phosphors to efficiencies of relatively high value, such that fluorescent lamps having an efficiency of approximately 80 lumens per watt are obtainable, such lamps do not complete, particularly in the outdoor lighting field, with high intensity are lamps because of their low brightness. Thus, for example, mercury arc lamps having efficiencies of only about 55 lumens per watt emit a much greater amount of light for a given volume, due to the greater intensity of the light therefrom. In order that a fluorescent light source be equivalent, in total light output, to an are light source, a much greater bulb area is required. Luminaires designed to accommodate such a larger area, being comparably larger, are prohibitively heavy and expensive. Thus, in the absence of a greater output per unit area from fluorescent lamps, such lamps are not competitive with presently available arc lamps.

Accordingly, it is an object of the present invention to provide fluorescent light sources which are greatly reduced in size without reducing the total light output therefrom.

Yet another object of the present invention is to provide high efficiency fluorescent lamps of small size emitting high brightness light.

Still another object of the present invention is the provision of high intensity fluorescent lamps which utilize far ultraviolet radiation as a means of phosphor excitation.

A further object of the invention is to provide improved fluorescent lamps for reprographic or photocopying purposes.

In one embodiment of the present invention, I provide very high intensity fluorescent lamp apparatus including a luminescent phosphor, which emits visible or near ultraviolet light when excited by far ultraviolet ionic radiation, coated upon the inner surface of the lamp envelope. Within the envelope, I provide a means for generating far ultraviolet ionic radiation including a vaporizable ionizable metal and a noble gas together with a pair of discharge electrodes. The metal is present in effective quantity and pressure, and has the characteristic, to emit far ultraviolet ionic radiation when an electric discharge of one ampere per square centimeter or higher density is established between the discharge electrodes, in accord with the operating mode of lamps of the invention.

The novel features characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following detailed description, taken in connection with the attached drawing in which the sole figure illustrates, in schematic vertical cross-section, a fluorescent lamp constructed in accord with the invention.

In the drawing, a typical lamp constructed in accord with the present invention, is represented generally at I0. Lamp 10 includes an evacuated, hermetically sealed envelope 11 having a pinch seal 12 at either end thereof and accommodating inleads l3 therethrough. Interior of envelope 11, a second, inner envelope 14 having an elongated tubular shape with a constricted central discharge section 15 between opposed bulbous ends 16 supports members 17, the ends of which are embedded in envelope 11. The ends of envelope l4 terminate in a pair of reentrant seals 18. inleads 13 pass through seals 18 and are terminated in a pair of filaments or discharge electrodes 19. Discharge electrodes 19, as illustrated herein, may, for example, be conventional fluorescent lamp electrodes and include a thermionic filament connected across inleads 13 and having associated therewith a pair of anode members 20 which function on alternate portions of an alternating current cycle with an oppositely disposed filament operating as cathode. Alternatively, particularly for high current density operation, hollow or hybrid cathode structures are utilized. A charge consisting of a quantity 21 of an ionizable vaporizable metal and a partial pressure of a noble gas are contained within inner envelope 14 to complete an operable, far ultraviolet light source as a portion of lamp 10. A fluorescent phosphor adapted to emit visible and/or near ultraviolet light when irradiated by far ultraviolet light is disposed over substantially all of the inner portions of the bulbous portion of outer envelope 11. Alternatively, in certain circumstances, the entire lamp may be enclosed in the outer envelope 11 and inner envelope 14 is not necessary.

Although phosphor 22 may be specifically designed to be particularly sensitive in the far ultraviolet ionic radiation of from 1,600 A. U. to 2,300 A. U., to be selectively excited by the ultraviolet ionic radiation means of the present invention, many conventional fluorescent phosphors which are normally excited by prior art fluorescent lamp discharges with atomic radiation of primarily 2,537 A. U., are even more sensitive to the far ultraviolet ionic radiation in the range of 1,600 to 2,300 A. U. than to 2,537 A. U. atomic radiation, and therefore, conventional fluorescent lamp phosphors may be used as phosphor 22 to advantage. Although such phosphors are well-known to those skilled in the art, for purposes of this disclosure, a calcium chlorofluorophosphate activated with antimony and manganese and having the formula, 3[Ca (PO .Ca(F Cl :Sb, Mn, may be utilized as the phosphor of the lamp 10 when light for illumination purposes is desired. Alternatively, in accord with that embodiment of the present invention in which reprographic lamps used for photocopying purposes are desired and an emission spectrum of the order of 3,600 to 4,200 A. U.

is desired, phosphor 22 may conveniently comprise calcium tungstate, magnesium silicate activated with titanium or calcium-magnesium silicate activated with titanium, for example, which phosphors are specifically sensitive to ultraviolet ionic emission and emit at wavelengths of approximately 3,600 to 4,200 A. U.

The far ultraviolet radiating means contained within inner envelope 14 may conveniently be adapted from any one of the lamps disclosed and claimed in my aforementioned previously co-pending patents. Briefly, in accord with the teachings of the aforementioned patents, far ultraviolet radiation is achieved by utilizing a fused quartz, high density alumina, or sapphire, for example, U.V. light transmissive discharge envelope containing a suitable pair of electrodes to carry the desired current density and maintaining within the discharge envelope a quantity or charge of a vaporizable ionizable metal which may be mercury, cadmium or zinc, together with a suitable inert gas which may be one of the noble gases, argon, neon, helium, xenon, or krypton. The operating parameters of the lamp are controlled both by optimizing cathode structure and lamp diameter and varying total input current to the lamp so as to cause a current density in excess of one ampere per square centimeter and preferably approximately 4 to 25 amperes per square centimeter to be established between discharge electrodes. The combination of appropriate impedance means 23 in the lamp input circuit (the discharge current within the lamp has a relatively low impedance) and the constriction of the lamp diameter to cause an increase in current density, are effective to convert conventional line voltage and only impedance limited current sources into the discharge current having this unusually high current density. The foregoing current densities are comparable to a normal current density for conventional fluorescent lamps of approximately 0.1 A per cm and a maximum current density for such lamps of approximately 0.25 A per cm The greatly increased densities used herein are achieved by varying the total lamp current by such well known lamp engineering techniques as decreased ballast impedance to achieve a total lamp current of approximately 2 to amperes as compared with a conventional fluorescent lamp current of approximately 0.3 to 1.5 amperes.

The heat generated by the high current discharge is controlled so as to cause the metal specie within the lamp to be vaporized to cause a partial pressure therein of approximately 10' to l torr of vaporized metal. The pressure of the inert gas is maintained at a value of approximately 1 to 20, but preferably 2 to 5 torr.

Under these conditions of vaporized metallic pressure and discharge current density, the metallic specie are excited to radiate principally ionic radiation in the far ultraviolet region of the electromagnetic spectrum. When the discharge specie is mercury and the noble gas utilized is helium, argon, or neon, the emission is principally from the 1,942 A. U. mercury ionic line which herein exhibits an unprecedented intensity for fluorescent lamps. When krypton is utilized as the noble gas and mercury is the radiating specie, the krypton enters into the radiation mechanism as a coexciting specie, such that the intensity of the 1,942 A. U. line is greatly enhanced and the mercury ionic 1,650 A. U. line is evident. When cadmium or zinc is the radiating specie, the preferable noble gas is xenon. When cadmium is utilized, the temperature of the radiation means is controlled, preferably by the inclusion of a shield about the electrode-containing ends of the inner discharge envelope 14 to allow the temperature to be raised to a value of approximately 200-400 C, and preferably approximately 250-350 C. Under these conditions, the emission is principally the 2,144 A. U. ionic line and the 2,265 A. U. ionic line. When zinc is the radiating specie, similar control of the bulb wall temperature in the vicinity of the discharge electrodes and the use of inner envelope 14 to control temperature and permit an operating temperature in the range of approximately 250450 C, and preferably 300400 C, result in the emission of far ultraviolet ionic radiation from the 2,026 A. U. ionic line and the 2,062 A. U. ionic line. Preferably, this emission is achieved with the xenon as the noble gas constituent. The control of the temperature of the ultraviolet radiating means envelope 14 in the vicinity of the discharge electrodes 19 controls the temperature of the coldest portion of the bulb wall and effectively controls the partial pressure of the active radiating metallic specie within the envelope. Such control is necessary in order that the proper pressure of radiating specie be present in order that the combination of the pressure and the current be such as to produce the desired radiation.

As is set forth hereinbefore, the ultraviolet radiation means envelope 14 is generally of a high melting point material such as fused quartz, high density alumina, or sapphire. The outer envelope, on the other hand, since it is insulated and isolated from any high temperature, may be any convenient soft glass or may be a more durable high temperature glass, if desirable. It need only be transmissive to visible and/0r near ultraviolet radiation. Inner envelope 14 must, on the other hand, he transmissive to far ultraviolet radiation.

Discharge electrodes 19 may, in certain instances, be conventional fluorescent lamp electrodes, as is illustrated in the drawing. This is sufficient for the lower portion of the current density range, for example, 1 to 5 A per cm operation of lamps in accord with the present invention. For higher currents, a thermionic hollow or hybrid cathode adapted to emit thermionically immediately upon excitation, to obtain the benefit of the high current density of the hollow cathode mode of operation, is preferable. A suitable hybrid thermionic hollow cathode is disclosed and claimed in the application of Harald Witting, Ser. No. 886,824, (now abandoned in favor of application Ser. No. 137,580 filed April 26, 1971, now U.S. Pat. 3,710,172) filed Dec. 22, 1969, and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference thereto for a more complete description thereof.

Basically, however, the Witting cathode is a hollow cathode having a pair of symmetrical curved surfaces disposed about the longitudinal axis of the cathode with a thermionic starter filament disposed therebetween. The split curved surfaces may, for example, be semicylindrical, concave, conical, or angularly disposed surfaces. The thermionic filament insures instantaneous starting, and the hollowcathode mode provides high current density. For discharge currents in excess of 20 amperes per square centimeter, it is desirable that high temperature are cathodes such as the coiled-coil or double coiled tungsten arc electrode, as is described in Reiling U.S. Pat. No. 3,234,421, be utilized. Current densities up to approximately 100 amperes per square centimeter may be achieved by utilizing such a desirable cathode structure.

Although, in the drawing, an interior envelope 14 is illustrated, such an envelope is not necessary in all circumstances. The reason for the double envelope structure is to provide a protective means for fluorescent phosphor 22 to protect the same against any deleterious effects of the high temperature and high current discharge and the ionized specie thereof. This is particularly desirable when cadmium and zinc are the radiating specie, since they require high temperatures, andis especially desirable, if not necessary, at the very high current densities which are achievable in some embodiments in accord with the present invention. For lower current densities, such as, for example, in the region of 5 amperes per square centimeter or less, particularly in the instance of utilization of a mercury discharge, there is little probability of damage to the fluorescent phosphor by the discharge. Under such circumstances, inner envelope 14 may be dispensed with and a single envelope containing both the ultraviolet emitting means and the phosphor is utilized. In such instances, the sole remaining envelope 11 should be constricted as discussed hereinafter and of sufficient refractory nature as to withstand the temperature of the arc and preferably Pyrex glass, or its equivalent, is utilized.

When the double envelope structure is utilized, it is desirable that a thermal buffer be provided between the inner envelope and the outer envelope. In the instance of the mercury discharge which operates at an operating temperature of approximately 15l20 C, a thermal buffer which controls the loss of heat therefrom is desirable. Accordingly, the inter-envelope space in such an embodiment is perferably filled with helium, argon, or nitrogen at a pressure of 50 to 500 torr. In the instance of cadmium and zinc as the radiating specie, on the other hand, it is desirable that the inter-envelope space be evacuated.

Due to the increased sensitivity of fluorescent lamp phosphors to the far ultraviolet radiation emanating from the far ultraviolet radiating means of the present invention, a much higher intensity, brighter light output is achieved from a given fluorescent phosphor in lamps of the present invention, as compared with conventional fluorescent lamps.

In designing any specific lamp for a specific use, it is normally desirable that the maximum intensity be obtained from the lamp, dictating the use of the maximum current density obtainable. The limiting factor or current density is the current which may be obtained from the electrodes available. Often the most desirable electrodes from a current carrying capacity viewpoint are not usable because of considerations such as space, heat dissipation limitations, cost, and the like.

Once a maximum available current (due to electrode configuration) is set, the maximum obtainable current density, which again may be limited by such factors as heat dissipation characteristics, is obtained by selecting the lamp discharge section diameter. Thus, with a given value of current, the more constructed the discharge section of the tube, the higher the current density. As a practical matter, utilizing the heaviest duty lamp electrodes presently available, lamps of the invention utilize discharge sections having maximum diameters of approximately 2 centimeters, although with the development of improved cathodes, the invention will be readily practiced with larger diameter discharge tubes.

In accord with the foregoing, when a conventional fluorescent lamp cathode (which has a current limit of approximately 5 amperes) is utilized in a lamp of the invention, in order to attain a current density of 4 amperes per square centimeter, a discharge tube inside diameter of 1.25 centimeters is used. On the other hand, using the same type cathode, in order to obtain a current density of 25 amperes per square centimeter, a discharge tubulation having an inside diameter of 0.5 centimeters is used. Using hollow or hybrid cathodes and higher currents, larger diameter discharge tubes may be used to obtain the same densities, due to the increased currents available.

One example of a specific lamp in accord with the invention used a phosphor coated section from a General Electric F-l5 T8CW lamp having an outside diameter of 25 millimeters and an inside diameter of 23 millimeters and a length of 26 centimeters coated with a calcium chlorofluoro-phosphate phosphor activated with antimony and manganese enclosed in an inner discharge envelope having an exterior diameter of 16 millimeters and an interior diameter of 14 millimeters with an inter-electrode spacing 23 centimeters, to form a structure as illustrated at 10. The lamp showed a visible light brightness of 8,000 foot lamberts. This was achieved with conventional fluorescent discharge electrodes at a total lamp discharge current pf 3.2 amperes yielding a current density of 2 amperes per square centimeter, and is to be contrasted with the maximum achievable brightness of from 2,500 to 3,500 foot lamberts achievable from conventional fluorescent lamps. Lamps constructed in accord with the present invention utilizing thermionic hollow cathodes and coiled coil or double-coil filaments are capable of producing intensities of the order of l5,000 to 50,000 foot lamberts due to the optimized high current characteristic of the improved electrodes.

In accord with the present invention, I have discovered that, unlike the ultraviolet light sources of the prior art which generally utilized mercury discharges at values of current and pressure of the mercury, such that the primary emission of the mercury spectrum was of the 2,537 A. U. atomic mercury line, the ultraviolet light sources in accord with the present invention utilizing high currents and low pressures, radiate ionic radiation that is within the range of l,6002,300 A. U. Thus, for example, when mercury is the radiating specie, the emission is the 1,650 and 1,942 ionic resonance lines. When the radiating specie is cadmium, the emission is from the 2,144 AU. and 2,265 A. U. ionic resonance lines. When zinc is the radiating specie, the radiation is the 2,026 A. U. and 2,062 A. U. resonance lines.

I have further discovered that although the emission of atomic radiation in lamps in accord with the prior art generally varies as a direct function of the discharge current, the emission of the ionic lines varies according to a relationship which may be stated by the formula,

where i is the current density, c is a constant and n equals 1.75 for the Hg-A discharge and n equals 2.0 for the Hg-Kr discharge. Thus, it follows that the emission of the ionic lines varies nonlinearly and is greater than linear emission almost by the square of the discharge current. In view of this relationship and the much higher current densities utilized in lamps according to the invention, I am able to attain a much greater intensity of radiation from the ionic lines than was ever achievable from the atomic radiation (principally 2,537 A. U.) of mercury, universally used in prior art fluorescent lamps. For this same reason, lamps in accord with the invention involve a much more intense stimulation of the fluorescent phosphors used and a resultant much greater intensity of fluorescent emission.

By the foregoing, I have disclosed new and improved high intensity fluorescent lamps utilizing a far ultraviolet excitation means and phosphors adapted to be excited by far ultraviolet radiation to the emission of near ultraviolet and visible light. Such lamps exhibit the pleasant color radiation of fluorescent lamps at high intensities comparable with mercury arc lamps.

While the invention has been disclosed herein with respect to certain preferred embodiments and specific examples thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Fluorescent lamp apparatus emitting exceptionally high intensity visible light and comprising:

a. a far U. V. emitting discharge means including:

a,. a visible light transmissive, hermetically sealed envelope having a constricted discharge tubulation section,

a a pair of discharge electrodes within said envelope for sustaining a gaseous discharge having a current density of greater than I ampere per square centimeter therebetween,

a and a filling within said envelope and including a noble gas and a vaporizable metal emitting high intensity far U. V. ionic radiation principally at specific ionic resonance lines of within the range of 1,650 A. U. to 2,300 A. U. under excitation by said discharge current,

b. a far U. V. ionic radiation sensitive visible light emitting phosphor deposited upon the inner surface of said envelope and adapted to emit high intensity visible light when irradiated by said far U.

V. radiation, and

c. contact means in contact with said discharge electrodes for connecting said discharge means to a current supply adapted to provide a discharge therein at said, current density.

2. The lamp apparatus of claim 1 wherein said far U. V. emitting means is enclosed in an evacuable hermetically sealed far U. V. transmissive envelope and the space within said visible light transmissive envelope exterior of said U. V. transmissive envelope constitutes a thermal buffer means.

3. The lamp apparatus of claim 1 wherein said filling within said discharge means envelope includes a vapor izable ionizable light emitting metal selected from the group consisting of mercury, cadmium and zinc and said noble gas is selected from the group consisting of helium, neon, argon, xenon and krypton and said exciting current is within the range of l to 25 ampere per square centimeter.

4. The lamp apparatus of claim 1 wherein the vaporizable metal is mercury and the noble gas is argon.

5. The lamp apparatus of claim 1 wherein the vaporizable metal is mercury and the noble gas is krypton.

6. The lamp apparatus of claim 1 wherein the vaporizable metal is selected from the group consisting of cadmium and zinc and the noble gas is xenon.

7. The lamp apparatus of claim 1 wherein said vaporizable metal is present in sufficient quantity as to yield a partial pressure in said discharge means during operation of from approximately l0 to 1.0 torr.

8. The lamp apparatus of claim 3 wherein said light emitting phosphor is sensitive to far U. V. excitation of wavelength less than 2,000 A. U.

9. Lamp apparatus as described in claim 2 and useful in reprographic uses and wherein said luminescent phosphor is sensitive to far U. V. radiation of wavelength of approximately l,6002,300 A. U. and emits radiation in the range of approximately 3,6004,200 A. U.

10. The lamp apparatus of claim 9 wherein said phosphor is selected from the group consisting of calcium tungstate, magnesium silicate and calcium-magnesium silicate. 

2. The lamp apparatus of claim 1 wherein said far U. V. emitting means is enclosed in an evacuable hermetically sealed far U. V. transmissive envelope and the space within said visible light transmissive envelope exterior of said U. V. transmissive envelope constitutes a thermal buffer means.
 3. The lamp apparatus of claim 1 wherein said filling within said discharge means envelope includes a vaporizable ionizable light emitting metal selected from the group consisting of mercury, cadmium and zinc and said noble gas is selected from the group consisting of helium, neon, argon, xenon and krypton and said exciting current is within the range of 1 to 25 ampere per square centimeter.
 4. The lamp apparatus of claim 1 wherein the vaporizable metal is mercury and the noble gas is argon.
 5. The lamp apparatus of claim 1 wherein the vaporizable metal is mercury and the noble gas is krypton.
 6. The lamp apparatus of claim 1 wherein the vaporizable metal is selected from the group consisting of cadmium and zinc and the noble gas is xenon.
 7. The lamp apparatus of claim 1 wherein said vaporizable metal is present in sufficient quantity as to yield a partial pressure in said discharge means during operation of from approximately 10 4 to 1.0 torr.
 8. The lamp apparatus of claim 3 wherein said light emitting phosphor is sensitive to far U. V. excitation of wavelength less than 2,000 A. U.
 9. Lamp apparatus as described in claim 2 and useful in reprographic uses and wherein said luminescent phosphor is sensitive to far U. V. radiation of wavelength of approximately 1,600-2,300 A. U. and emits radiation in the range of approximately 3,600-4,200 A. U.
 10. The lamp apparatus of claim 9 wherein said phosphor is selected from the group consisting of calcium tungstate, magnesium silicate and calcium-magnesium silicate. 